This section is intended to provide a background or context to the invention disclosed below. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived, implemented or described. Therefore, unless otherwise explicitly indicated herein, what is described in this section is not prior art to the description in this application and is not admitted to be prior art by inclusion in this section.
A filter is composed of a number of resonating structures and energy coupling structures which are arranged to exchange RF energy between themselves and the input and output ports. The pattern of interconnection of these resonators to one another and to the input/output ports, the strength of these interconnections and the resonant frequencies of the resonators determine the response of the filter.
During the design process for a filter, the arrangement of the parts, the materials from which the parts are made and the precise dimensions of the parts are determined such that an ideal filter so composed will perform the desired filtering function. If a physical filter conforming exactly to this design could be manufactured, then the resulting filter would perform exactly as intended by the designer.
However, in practice the precision and accuracy of manufacture of both the materials and the parts are limited, and this results in errors in resonant frequencies and coupling strengths which in turn cause the filter response to differ from that predicted by the ideal filter model. Often, this departure from the ideal response is sufficiently large to bring the filter outside of its acceptable specification. As a result, it is desirable to include in the filter design some means of adjusting the resonator frequencies and couplings to bring the filter response within the required specification.
A common way to accomplish this is to include tuning screws or other devices, such as are well known in the art. An alternative way often used in small ceramic monoblock filters is to remove selected portions of the metallization on the exterior of these filters, and possibly portions of ceramic as well, to accomplish the tuning.
Most filters are manufactured as completed units and the tuning process then performed on the entire filter. Since the many adjustments on the filter interact strongly with one another, the tuning procedure is often quite complicated, and requires a skilled operator.
An alternative tuning method is to build the separate resonator parts, tune them individually to a specification calculated for the separated parts from the ideal filter model, and then assemble them into the final filter. Since the individual parts are simple compared with the fully assembled filter, the tuning procedure for these individual parts can also be made very simple. This minimizes the need for skilled operators to tune the filters. This procedure also provides the benefit of either reducing or entirely eliminating the tuning process for the assembled filter.
In many cases it is sufficient to adjust only the resonant frequencies of the resonator parts because the manufacturing precision and accuracy are good enough to bring the coupling strengths within the required range to allow the assembled filter performance to be within specification. In such cases adjustment of the frequencies alone is all that is required to tune the individual parts.
To allow pretuning of the individual parts, both methods of measurement of the frequencies and methods of adjustment of the frequencies are required.
A tuning method may include the manipulation of a tuning device or structure included as part of the resonator, such as a tuning screw or deformable metal part. Alternatively, a method may comprise an operation performed on the resonator, such as the removal of material from a selected region. The method may also comprise a combination of these, or any other means or process which can alter the resonant frequencies of the resonator part.
A tuning physical adjustment (commonly abbreviated more simply as “adjustment”) can then be defined as one or more manipulations of tuning structures and/or one or more operations causing one or more of the resonant frequencies to be altered. For instance, such physical adjustment includes but is not limited to removal of material from a surface/face, drilling of holes, adjustments of screws, and/or denting of material.
In cases where the parts include multimode resonators, the tuning methods for the parts will need to be able to independently adjust the resonant frequencies of the several modes of the resonator. For example, if the multimode resonator has three modes requiring independent adjustment, then at least three independent tuning adjustments will be required. It is a common situation with multimode resonators that an individual adjustment causes more than one of the mode frequencies to change. As a result, there is not a one-to-one correspondence between a single adjustment and a frequency change in a single mode.
What is needed to allow the multimode part to be easily tuned is a method to calculate the required adjustments to effect the desired changes in the set of mode frequencies.